Radio Frequency (RF) or microwave power supplies (hereafter “RF power supplies”) are widely used in semiconductor and industrial plasma processing equipment to generate plasmas in a process chamber. Plasma processing is used for a wide variety of applications, including etching of materials from substrates, deposition of materials on to substrates, cleaning of substrate surfaces, and modification of substrate surfaces. The frequency and power levels employed vary widely, from about 10 kHz to 2.45 GHz and from a few Watts to as much as 100 kW or greater. For semiconductor processing applications, the range of frequencies and powers presently used in plasma processing equipment is somewhat narrower, ranging from about 10 KHz to 2.45 GHz and 10 W to 30 kW, respectively.
Prior art RF power supplies used in plasma sources for plasma processing equipment generally have expensive and complex power generation and delivery systems. These plasma sources require a precision RF power generator, a power delivery system, a matching network, and metrology (measuring) equipment. In addition, precision instrumentation is usually required to control the actual power reaching the plasma The cost of these prior art RF power supplies can be a considerable fraction of the total system cost.
The impedance of plasma loads can vary considerably in response to variations in gas recipe, plasma density, delivered RF power, pressure and other parameters. The RF supply can deliver power to the plasma in a number of different ways. This can include inductive coupling via an antenna structure, capacitive coupling, launching a wave, exciting a resonant cavity, etc. The RF supply generally requires proper matching to the load impedance.
An antenna typically has a primarily inductive load impedance, with a smaller resistive component. In contrast, a sample holder or “chuck” typically presents a primarily capacitive impedance, also with a smaller resistive component. RF power can be delivered to these loads via an impedance matching network.
Most prior art RF generators for plasma processing equipment are designed to have a standard fifty-ohm output impedance. A matching network is required because the load represented by the process chamber and the plasma can vary widely and rapidly, causing mismatches in impedance between the standard fifty-ohm output impedance of the RF generator and the input of the load. A mismatch in the impedance of the generator and the plasma source causes great stress on electronics devices in the RF generator and the matching network and can cause premature failure because of either electrical or thermal stress or both.
Consequently, the reliability of prior art RF generators and matching networks is relatively low and is considered to be below desired standards of the semiconductor industry. The relatively low reliability increases the total cost of ownership (COO) of the plasma processing tool, since time must be spent in diagnosing failures and repairing or replacing defective RF equipment. Impedance mismatch also causes the power delivered to the plasma to vary, which can cause process inconsistency both within a chamber for successive substrates and among similar chambers.
Prior art matching networks are positioned in the power delivery system between the output of the RF generator and the input of the process chamber. The matching network provides a means of matching the output impedance of the generator to the input impedance of the process chamber. The matching network may contain fixed elements only, or it may contain elements such as variable capacitors and variable inductors, which can allow dynamic impedance matching of the generator to a changing load.
In recent years it has become common to use frequency tuning to carry out dynamic impedance matching. The matching network for dynamic impedance matching systems employing frequency tuning typically contains only fixed elements. Changes in load impedance can be accommodated by slightly varying the RF frequency. Dynamic impedance matching generally provides faster tuning speed, higher reliability, lower cost, and lower size. The dynamic tuning range, however, is relatively low.
A matching network having fixed reactive elements can be used to transform a reactive load to a load that appears purely resistive and can also be efficiently driven by a variable frequency RF supply. This approach, however, would typically require a very wide frequency range, e.g. +/−30%, because the load impedance can vary widely, e.g., +/−200%. Such a wide frequency range is unacceptable for processing reasons and also because of potential interference with other equipment protected using narrow-band filters.
A matching network of variable vacuum capacitors driven by servo-motors may accommodate a widely varying load. Mechanical motors, however, are relatively slow, while vacuum variable capacitors are expensive.
An approach for faster mechanical tuning is described in U.S. Pat. No. 5,654,679 to Mavretic, et al. This approach employs PIN diodes or relay switches to add or remove capacitors as participants in a matching network to maintain a somewhat constant load impedance, as presented to the RF supply.
This approach has several disadvantages. The matching network is complex because it requires many switches. PIN diodes are susceptible to breakdown and are relatively expensive. Switching is performed in a discontinous fashion; a PIN diode or relay has a binary state - either on or off. This can cause discontinous jumps in the resonant frequency and impedance seen by the RF supply, as well as off-resonance operation of the RF supply while the resonant frequency is re-established by a feedback control loop. Off-resonance operation can cause significant stress on field effect transistor (“FET”) switches. Reduction of these problems requires, for example, use of many PIN diode switches, each requiring an associated capacitor and driving circuitry.